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Sunday, August 14, 2022

Explained: The black hole at the centre of Milky Way, photographed for the first time

The photograph provides evidence that the object at the centre of the Milky Way, which has been a subject of speculation for decades, is indeed a black hole.

By: Explained Desk | New Delhi |
Updated: May 17, 2022 12:41:51 pm
This image released by the Event Horizon Telescope Collaboration on Thursday shows a black hole at the center of our Milky Way galaxy. (Photo: AP)

The first photograph of Sagittarius A* (pronounced as sadge-ay-star), a supermassive black hole about 26,000 light years from Earth and situated at the centre of the Milky Way, was revealed by astronomers of the Event Horizon Telescope (EHT) on Thursday. More than 300 researchers from 80 institutions collaborated on this project.

In April 2019, astronomers captured the first ever photograph of a black hole M87, which was located in a distant galaxy called Messier 87.

We take a look at how the picture, which is regarded as a technical tour de force, was captured.

What are black holes?

A black hole is formed when stars collapse, leading to a space in the universe with an escape velocity — the speed at which an object must travel to override a planet or an object’s gravitational force. For instance, for a spacecraft to leave the surface of the Earth, it needs to be travelling at a speed of about 40,000 km per hour which is so great that even light cannot escape it.

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Because light cannot get out, black holes are invisible and can only be tracked with the help of spatial telescopes and special tools. The light cannot go out because the gravity inside a black hole is very strong as a result of a lot of matter being squeezed into a small space.

In 2020, the Royal Swedish Academy of Sciences decided to award one half of the year’s Nobel Prize in physics to Roger Penrose and the other half jointly to Reinhard Genzel and Andrea Ghez for furthering the understanding of black holes, the most “enigmatic” objects in the universe.

Genzel and Ghez discovered that an invisible and an extremely heavy object governs the stars’ orbit at the centre of the Milky Way. This extremely heavy object has the mass equivalent to 4 million solar masses and is packed into an area about the size of our solar system.


Essentially, their work tells us that at the centre of the Milky Way lies an invisible supermassive object, of which a black hole provides a reasonable explanation. Physicists have been suspecting the existence of a black hole at the centre of our galaxy for over 50 years now.

In order to see through to the middle of the Milky Way, Genzel and Ghez worked on developing methods and used some of the world’s largest telescopes.

By observing the orbits of the stars’ that are closest to the centre of the Milky Way, the physicists think that the black hole could most likely be hiding in Sagittarius A*, a source of radio waves around which all stars in the Milky Way orbit.


Why and how was the blackhole photographed?

The photograph provides evidence that the object at the centre of the Milky Way, which has been a subject of speculation for decades, is indeed a black hole. Since the black hole is far away from Earth, to the astronomers looking at the sky, it appeared to be about the same size as a donut on the Moon.

To take its photograph, the team created a powerful Event Horizon Telescope after which Sagittarius A was observed on multiple nights and data was collected for hours in a row, an exercise similar to using a long exposure time on a camera.

A statement released by EHT said that scientists are excited about having images of two black holes that are of very different sizes, “which offers the opportunity to understand how they compare and contrast.”

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First published on: 14-05-2022 at 05:46:40 pm
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